Using lasers for acoustic measurement

The wizards over at the UK National Physical Laboratory have figured out a way to map sound fields without microphones. The technique they have come up with uses laser vibrometers developed for underwater sonar analysis.

See below for an excerpt from the press release and a video explaining the technique. Cool stuff huh?

From the press release:

High-performance loudspeaker manufacturers have been able to improve sound quality dramatically over the years, but still face the issue of dead spots. While loudspeakers can be designed to deliver the full frequency range of audible sound, it is difficult to achieve a smooth frequency output in all directions. Dead spots are caused by destructive interference as a result of radiating sound waves overlapping and cancelling each other out. The biggest issue being where the sound is radiating from two or more sources, which commonly occurs in the mid-frequency ranges where both the ‘woofer’ and ‘tweeter’ loudspeaker cones are both active. This creates areas where the frequency response of the loudspeaker is less smooth, and sound quality is diminished.

Determining the nature of these dead spots has proven difficult until now. High accuracy acoustic measurements can be made using a microphone, but to build up a picture of the spatial distribution of the sound many point measurements are required within the 3D space. Manufacturers can conduct computer-aided simulations, but these can prove inaccurate to the actual loudspeaker performance through the variability of the manufacturing process.

Now the National Physical Laboratory (NPL), the UK’s Measurement Institute, has developed a solution. The new laser-driven technique allows remote, non-invasive and rapid mapping of sound fields, which will provide loudspeaker manufacturers with reliable data on which to design their technology. The technique builds on a piece of technology developed for the study of mechanical vibration; the laser vibrometer, and on research for its application to the 3D characterisation of underwater sonar arrays. This NPL work has shown that in air, the acousto-optic effect, the resulting optical phase change of light as it passes through an acoustic field, is significant enough to be detected. To measure the acoustic output from the loudspeaker, the laser is positioned to the side of the loudspeaker and is rapidly scanned through a series of points in front of the loudspeaker, being reflected back to the laser vibrometer by virtue of a retro-reflective mirror on the other side. By measuring the laser as it returns to its source, the technology can rapidly provide spatially distributed phase shift data, enabling an image, or video, of sound propagation around the source to be constructed.

Ian Butterworth, project lead at NPL, said:

“This is a significant breakthrough for loudspeaker manufacturers. The main applications are likely to be for high-end in-home loudspeaker manufacturers who want their products to deliver the perfect sound experience”

One thought on “Using lasers for acoustic measurement”

I agree, that is pretty awesome. 100k frames per second! Given that the laser presumably has to scan many lines per frame, that would be incredibly fast scanning. I wonder if that isn't a precise number… either that or they have a micromirror array to scan the laser, because I doubt you could go that fast with conventional mirrors and motors.

I also wonder if the thing can scan a larger area so as to include behind/above/under the speaker as well. Also, you'd want to have a vid in the horizontal plane as well. Probably it's already expensive to do this in one axis so I bet the intention is that you change the speaker orientation and run another sweep.